linux-kernel - Re: [PATCH v2 10/10] kernel: might_fault does not imply might

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Message-ID: <1368979579.6828.114.camel@gandalf.local.home>
Date:	Sun, 19 May 2013 12:06:19 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	David Howells <dhowells@...hat.com>,
	Hirokazu Takata <takata@...ux-m32r.org>,
	Michal Simek <monstr@...str.eu>,
	Koichi Yasutake <yasutake.koichi@...panasonic.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, linux-m32r@...linux-m32r.org,
	linux-m32r-ja@...linux-m32r.org, microblaze-uclinux@...e.uq.edu.au,
	linux-am33-list@...hat.com, linuxppc-dev@...ts.ozlabs.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep

On Sun, 2013-05-19 at 16:34 +0300, Michael S. Tsirkin wrote:

> Right but we need to keep it working on upstream as well.
> If I do preempt_enable under a spinlock upstream won't it
> try to sleep under spinlock?

No it wont. A spinlock calls preempt_disable implicitly, and a
preempt_enable() will not schedule unless preempt_count is zero, which
it wont be under a spinlock.

If it did, there would be lots of bugs all over the place because this
is done throughout the kernel (a preempt_enable() under a spinlock).

In other words, don't ever use preempt_enable_no_resched().

-- Steve


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